1. Field of the Invention
The present invention relates to an image forming apparatus that irradiates an image forming member with electrons emitted from an electron emitting element to form an image and in particular, to an image forming apparatus provided with an electron emitting element having such a threshold voltage to be applied to an anode that starts to cause electron emission.
2. Description of the Related Art
A field emission type (hereinafter referred to as “FE type”) electron emitting device has been known as an electron emitting device. The application examples of this FE type electron emitting device include an electron source constructed of many FE type electron emitting devices arranged on a board and an image forming apparatus such as a flat panel display using the electron source. To realize an image forming apparatus like this, a board having an electron source formed thereon, a phosphor, and an anode are arranged in such a way that the phosphor and the anode are opposite to the board and electrons emitted from the electron emitting device are collided to the phosphor on the anode side to make the phosphor emit light.
A carbon base material that has a small work function for emitting electrons and a small threshold voltage receives attention as a material especially suitable for the electron emitting element of the electron emitting device. Examples using a carbon base material for an electron emitting element are disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-243218 (specification of U.S. Pat. No. 6,437,503), JP-A Nos. 2000-251783, 2000-268706 (specification of U.S. Pat. No. 6,400,091), JP-A Nos. 2002-100279, and 2003-031166.
In these patent documents, fullerene, diamond, diamond like carbon (DLC), and carbon nano tube (CNT) are used as an electron emitting element.
One example of a method for driving an electron emitting device like this will be briefly described in the following.
FIG. 27A is a schematic view to show an electric potential distribution in a drive state where electrons are emitted and FIG. 27B is a schematic view to show an electric potential distribution in a state where electron emission is stopped. Here, FIG. 27 is a schematic view only to describe the content of the invention and the ratio of sizes of parts are not necessarily correct.
FIG. 27A shows a drive state where a voltage is applied to an anode 906 so as to make an electron emitting element 905 on a cathode 902 generate an electric field larger than a threshold electric field to start emitting electrons, thereby causing electron emission.
On the other hand, FIG. 27(b) shows a state where a negative voltage is applied to a gate 904 to make an electric field intensity in the vicinity of the electron emitting element 905 smaller than a threshold electric field required to emit electrons, thereby stopping the electron emission.
In this example, when the electron emission is stopped, as shown in FIG. 27(b), the electric potentials of the cathode 902 and the electron emitting element 905 are brought to 0 V and the electric potential of the gate 904 is brought to a negative value, so that it is clear that the gap between equipotential surfaces in the vicinity of the electron emitting element 905 becomes wide and that the electric field becomes small.
In this regard, the voltage applied to the gate 904 so as to stop the electron emission is determined by the threshold electric field of the electron emitting element 905, the electric field intensity by anode voltage, and designs such as the size of the electron emitting element 905, the distance between gate and cathode, and a gate size.
In a method for driving the electron emitting device described above, the electron emission is caused only by applying a voltage to the anode and a voltage of stopping electron emission is applied between the cathode and the gate to intercept the electron emission to control the electron emission. Hence, it is possible to drive and control the electron emitting device with ease and at low voltage.
By the way, in the case of a display to which the structure of the electron emitting device and the drive method described above are applied, an emitted electron is headed to the anode nearly directly above the electron emitting element. For this reason, there are cases where positive ions ionized by the electron beam fall onto the electron emitting element to have an effect on the life of the electron emitting device. Further, at the time of power being turned off, including an accidental power failure, there are cases where the entire surface of a display screen emits light until electric charges accumulated in the anode are discharged, which causes a user to feel abnormal.
Further at the time of power being turned on or off or power failure, there is easily brought an unstable state where a voltage sufficient large enough to stop electron emission from the electron emitting element can not be applied between the cathode and the gate. As a result, there are cases where a predetermined voltage is applied only between the cathode and the anode to cause the unintended light emission of the display screen, as described above. Hence, it is required to take measures against these problems.